During a 2012 deposition, a lawyer for the N.F.L. argued that the league was adamantly opposed to sports gambling because it would “negatively impact our long-term relationship with our fans, negatively impact the perception of our sport across the country.”

Today, not so much.

For Sunday’s Super Bowl, 31.4 million Americans are expected to place $7.6 billion in legal bets, both records and increases of 35 and 78 percent over last year, respectively, according to the American Gaming Association.

This past year, partnerships with sports gambling companies and casinos represented a significant chunk of the N.F.L.’s record $1.8 billion in sponsorship revenue, with virtually every big name — including DraftKings, FanDuel, Caesars, BetMGM and PointsBet — getting a piece of the action.

And earlier this week, the N.F.L. reached its first sportsbook deal in Canada, in anticipation of the introduction of regulated sports betting in Ontario in April.

“It’s a tectonic shift — it’s massive in terms of taking one side of one issue, and benefiting from the other side 10 years later,” said Max Bichsel, the vice president of NewYorkBets.com, a sports betting research and analysis company.

For those who may have only recently started to pay attention to sports betting — perhaps because of the barrage of commercials during the N.F.L. playoffs and the Olympics featuring the Manning family and other former athletes — here is a guide to how the N.F.L. did an about-face in just a decade.

Just how opposed was the N.F.L., anyway?

Very.

For decades, the N.F.L. feared that legalized gambling would commingle with match-fixing and corruption and hurt the integrity of the sport. One of the defining scandals in the N.F.L.’s pre-merger days was the 1963 suspension of two major stars, the Hall of Famers Paul Hornung of the Green Bay Packers and Alex Karras of the Detroit Lions, for betting on league games and associating with gamblers or “known hoodlums.”

That reluctance only intensified in 1992, when President George H.W. Bush signed the Professional and Amateur Sports Protection Act, or PASPA, which banned sports wagering in most states, with Nevada being the most notable exemption.

N.F.L. players were prohibited from participating in events that took place at or were sponsored by casinos. The best-known example, perhaps, was when the former Dallas Cowboys quarterback Tony Romo was blocked in 2015 from attending a fantasy football event at a convention center attached to a casino in Las Vegas.

When did the N.F.L. begin to have second thoughts?

In 2014, Adam Silver, the N.B.A. commissioner, wrote an opinion piece in The New York Times supporting efforts to regulate and legalize sports betting. After all, Silver argued, underground sports betting worth an estimated $400 billion annually was already occurring, and times had changed since PASPA was enacted. Lotteries and casinos had become ubiquitous.

The N.F.L. and everyone else took notice.

“This was not a light-switch moment — this was very slow and very steady,” said John Holden, a business professor at Oklahoma State University who has written extensively about sports gambling. “The N.F.L. watched as the other three leagues jumped, and then the N.F.L. said, OK.”

Just as important was the fact that two of the most prominent N.F.L. team owners, Robert Kraft of the New England Patriots and Jerry Jones of the Dallas Cowboys, were early and enthusiastic investors in DraftKings and fantasy sports. And while the N.F.L. challenged New Jersey’s new sports betting statute in 2012 — a case decided by the Supreme Court — individual teams signed sponsorship deals with sportsbooks companies, which began spending big on television spots during the N.F.L. season.

Those contradictions were never more evident than in 2017, when the league’s owners voted overwhelmingly to allow the Raiders to move to Las Vegas from Oakland, Calif., and local officials paid $750 million in construction costs for a new stadium right off the Strip, said David G. Schwartz, an Atlantic City, N.J., native and longtime chronicler of gambling history who is now the ombuds at the University of Nevada Las Vegas.

The Supreme Court makes a decision.

The seismic change in sports betting, of course, occurred in 2018, when the Supreme Court struck down the 1992 law. States that had anticipated the decision, like New Jersey, got an early jump on sports betting. Leagues and teams became even more emboldened to collaborate with gambling-related businesses, including casinos and betting apps.

“When the world is changing, you want control,” said Oliver Hahl, a Carnegie Mellon University business professor who has studied sports authenticity and organizational theory.

To be sure, the N.F.L. and other leagues tried to exert more control by arguing, chiefly in statehouses around the country, that there should be some taxes or fees that went to the leagues, and not the sportsbooks. But the leagues have had only limited success, said Chris Grove, a distinguished fellow at U.N.L.V.’s International Center for Gaming Regulation, and a partner emeritus at Eilers & Krejcik Gaming, a research and consulting firm.

More than 30 states have authorized sports wagering since the Supreme Court decision, and California could join them later this year. Meanwhile, New York, where the N.F.L. has its main headquarters, has quickly become the nation’s largest sports betting market, topping New Jersey, after just four weeks of accepting mobile bets.

“In a few short years, the very idea of sports betting has been transformed from a taboo and sub rosa vice into a major business opportunity,” according to a recent report from MoffettNathanson, a research firm. “All the leagues have flipped positions to now embrace sports gambling with exclusive partnerships. Following the money and consumer interest, media companies have struck many deals to align themselves with various sports betting operators.”

What about the future?

Football is the most popular sport for bettors, and the N.F.L. continues to worry about the integrity of the game. In 2019, the league suspended Josh Shaw, an injured Arizona Cardinals defensive back at the time, for betting on N.F.L. games. And any scandal that ensnares players, coaches or referees on the order of Tim Donaghy, a former N.B.A. referee who went to prison for betting on games, would be devastating.

The N.F.L. has also been under pressure to tackle compulsive gambling behavior. It started a campaign to encourage responsible gambling at the beginning of the current season, through a $6.2 million partnership with the National Council on Problem Gambling.

Still, more controls are needed, especially on in-game sports betting, which feeds into a compulsive gambler’s desire for more and faster opportunities to bet, said Keith Miller, a professor at Drake University Law School. Miller spoke during a panel Wednesday on the ethics of legal sports gambling organized by Baruch College’s Zicklin School of Business.

What the N.F.L. and the country should keep in mind, said Holden, who was also on the panel, is the experience of the United Kingdom and Europe, where legal sports betting has been more established: Both have witnessed serious gambling addiction issues.